Choline, also known as choline base, (2-hydroxyethyl)trimethylammonium hydroxide, is a well-known organic base suitable for a variety of uses.
For example, aqueous solutions of choline base are useful in connection with electronic applications such as positive photoresist developing agents, as anisotropic etching agents, and as washing agents for silicon wafers. U.S. Pat. No. 4,172,005 (Muraoka et al.) describes the use of an aqueous solution of trihydrocarbon-substituted (hydroxyhydrocarbon-substituted) ammonium hydroxide as an anisotropic etchant for semiconductor substrates; U.S. Pat. Nos. 4,239,661 and 4,339,340 (Muraoka et al.) describe the use of an aqueous solution of trialkyl(hydroxyalkyl) ammonium hydroxide as a cleaning agent for semiconductor wafers, as an etchant for metal layers used as wiring, and as a developer and stripper for positive working photoresist films; and U.S. Pats. Nos. 4,294,911 and 4,464,461 (Guild) describe the use of an aqueous solution of quaternary alkanol ammonium hydroxide as a developing agent for positive-working photoresists and positive-working lithographic printing plates.
Such aqueous solutions typically have choline concentrations between about 0.01% and 40% by weight choline; and are typically prepared and shipped as concentrates having a choline content of at least 10% by weight, the concentrates being diluted to concentrations typically between 1% and 5% by weight choline for use.
Solutions of choline base in lower alcohols, typically methanol, have been commercially available for many years. In concentrations as high as 50% by weight choline, they have been suggested for use as replacements for (alcoholic) solutions of common alkalies, and as catalysts, curing agents, hydrolytic agents, neutralizing agents, and solubilizing agents, and for pH control.
Use in the electronics area requires that there be no residue following the normal post bake period because even traces of impurities such as alkali metals would interfere in the operation of the electronic circuits. Accordingly, impurity specifications for choline base to be used in the electronics industry are very strict. Typically such specifications are, based upon contained choline base, less than 10,000 ppm Cl, Br, I, or carbonate and less than 15 ppm each of Li, Na, and K. It is understood, however, that it is advantageous to the electronics fabricator to employ choline base in which the above mentioned impurities approach zero.
While compositions containing a choline base developing agent are excellent developers for resists of the quinone diazide type, and have other uses as set forth above, they suffer from a serious problem in that gradual decomposition of the choline base, with consequent darkening of the composition and changes in its performance as a developer, tends to take place. A solution to this problem is provided in U.S. Pat. No. 4,294,911. The solution is the incorporation in the developing composition of the stabilizing concentration of a sulfite, for example, ammonium sulfite. Sulfites function very effectively as stabilizing agents, that is, as agents which serve to prevent or retard darkening of the composition and changes in its performance as a developer for quinone diazide compositions. As explained in U.S. Pat. No. 4,294,911, it is believed that quaternary alkanol ammonium hydroxide developing agents can decompose to form aldehydes and that the aldehydes can polymerize to form polyaldehydes, which are highly colored materials that result in a gradual darkening of the composition. Depending on the particular conditions involved, the polyaldehydes can be dimers, trimers, oligomers or low molecular weight polymers. Sulfites are believed to form adducts with the aldehydes and thereby prevent the formation of the polyaldehydes. U.S. Pat. No. 4,425,202 (Sullivan) describes the use of sulfites in the electrolytic manufacture of choline base from choline salts, where the sulfite is stated to provide color stabilization.
While the use of sulfites as stabilizing agents repesents an important advance in the art, it is subject to a serious disadvantage which has hindered the commercial utilization of developing compositions containing quaternary alkanol ammonium hydroxide developing agents. Such developing compositions are typically prepared, shipped and stored as concentrates and are diluted by the user to the proper concentration. The activity of these developing solutions is, in part, dependent on pH and the optimum pH in a given situation will depend on various factors, such as the particular ingredients in the light-sensitive composition, the particular ingredients in the developing composition, the type of equipment utilized in exposing the light-sensitive composition, the development temperature, and so forth. It has been found that if a sulfite is used as a stabilizing agent, it interferes with the titration and renders it impossible to accurately determine the total solution alkalinity by a titration process.
In U.S. Pat. No. 4,464,461, the use of a semicarbazide as a stabilizing agent is proposed for aqueous quaternary alkanol ammonium hydroxide solutions. Semicarbazides are stated to avoid the disadvantage of interference with alkalinity determination, while providing adequate stability to enable storage and shipment of concentrated solutions, and not interfering with use as a developer. Semicarbazides are, however, relatively expensive.
The disclosures of each of the documents referred to herein are incorporated herein by reference.
Methanolic choline base is generally, at the 50% by weight choline level, stabilized by the addition of ethylenediamine, typically at between 1000 and 2500 ppm of the total solution.
It would be desirable to provide a stabilized choline base solution in water and/or lower alcohols which provides the desired stability against decomposition and color formation, while still being relatively inexpensive, and not interfering with the intended use of the solution.